Dry compositions for deoxidizing and desmutting aluminum and aluminum alloys



United States atent fiice 3,373,114 Patented Mar. 12, 196.8

3,373,114 DRY COMPOSITIONS FOR DEOXIDTZING AND DESMUTTING ALUMINUM AND ALUMTNUM ALLOYS John J. Grunwald, New Haven, Conn., assignor to Mac- Dermid Incorporated, Waterbury, Conn., a corporation of Connecticut No Drawing. Continuation-impart of application Ser. No. 313,785, Oct. 4, 1963. This application Jan. 3, 1967, Ser. No. 606,575

4 Claims. (Cl. 252--136) The present invention relates to the surface preparation of aluminum and alloys for subsequent metal finishing operations such as painting, anodizing, plating, bright dipping, welding, chromating, etc. This application is a continuation-in-part of application Ser. No. 313,785 fiied Oct. 4, 1963 now abandoned and of application Ser. No. 104,816, filed Apr. 24, 1961, now Patent No. 3,140,203.

It is an important procedure in the aluminum finishing industry, prior to the finishing operations mentioned above, to remove the ever-present oxide film which forms on the surface of aluminum alloys on atmospheric exposure. A typical method employed in the past by which this is achieved involves immersing the previously degreased aluminum object in a suitable deoxidizing solution, which usually contains oxidizing agents such as chromic acid, chromates or dichromates, nitric acid or a mixture of these materials in well defined proportions. Their function is to replace this tenacious oxide film on the aluminum alloy by another film more suitable for subsequent treatments.

The use of chromic acid or any other chrome base deoxidizing solution in the finishing industry creates waste disposal difliculties, since these formulations must be treated chemically before they can be disposed of without causing pollution problems. Also the use of chromic acid in certain finishing shops may cause contamination of rinsing waters, bright dips, anodizing and plating solutions. Furthermore, the use of nitric acid is always accompanied by obnoxious fumes which makes for conditions uncomfortable to personnel and hazardous to their health, as well as causing corrosive action on processing equipment and materials.

Another procedure frequently used heretofore in the aluminum finishing industry as a preparatory step, prior to the finishing treatments, consists in etching the aluminum alloy in a suitable alkaline solution, thereby removing substantial amounts of surface metal. During this etching process, certain alloying elements of the aluminum, which are insoluble in the etching solution, will cover the surface in the form of a loose coating forming an unsatisfactory base for further treatment, and care must therefore be taken to see that this loose coating is completely removed. This phenomenon is known in the industry as smut formation, and removal of the smut is commonly referred to by the term deoxidizing in practice and is so used in the present specification and appended claims.

In the above-mentioned Patent No. 3,140,203, there is disclosed a novel aluminum deoxidizing solution comprising in combination a strong inorganic acid, more particularly sulphuric acid, and a soluble persulfate. Ammonium persulfate or one of the more readily available alkali or alkaline earth metal salts of persulfuric acid are specifically recommended. Providing the acidity of the solution is maintained at a pH not in excess of 2.0, excellent deoxidation at room temperature is obtained for all alloys of aluminum except those with high silicon content. Even where those alloys do have a high percentage of silicon, however, they can be handled in such system by adding a fluoride anion.

In the solution described above, the persulfate reacts with aluminum, or its alloying elements in the smut, and is reduced to aluminum sulphate. Thus, unlike prior de oxidizing baths, the exhausted solution contains no objectional components which give rise to waste disposal problems. The invention here disclosed differs from that claimed in the aforesaid Patent No. 3,190,203 in that it is directed specifically to dry, powder compositions from which an oxidizing solution of the type disclosed in the aforesaid patent may be prepared. The powder compositions provide advantages over the liquid equivalents in regard to easier handling and lower shipping weights, but more importantly and quite surprisingly produce substantial improvement in certain other respects as will more fully appear herein.

One such improvement lies in the greater stability of the powder compositions during storage until sold and shipped for use. It is well-known, of course, that the peracids are troublesome, presenting certain definite hazards in respect to explosive decomposition. A concentrated sulphuric acid solution of a persulfuric acid salt, for example, upon standing for sufiicient time as in a warehouse, can and may deteriorate with the formation of Caros acid (H according to the following mechanism:

Thus highly concentrated solutions of persulfuric acid may become quite dangerous, especially when stored in certain metallic containers over extended periods of time, since they favor formation of highly unstable hydrogen peroxide.

An additional advantage of the dry powder composition is realized by avoidance of the hazard involved in preparing the treatment bath from the concentrated solution as commonly supplied for shipment, which hazard arises from the highly exothermic nature of the reaction produced by addition of concentrated sulfuric acid to Water.

Another and most significant improvement resulting from the dry powder admixture of the operative components is a most significant and substantial increase in the useful life of a treatment solution prepared from the powder composition vis-a-vis the liquid equivalents. The improvement noted here is remarkable and quite unexpected, giving expected useful life of a treatment bath measured in months up to a year or more in some cases, (providing of course normal replenishment is made for unavoidable drag-out and consumption) as compared to a bath life expectancy of but a few weeks at best when using the liquid equivalents when initially preparing the bath. The reason or mechanism for this unusual stability is not clearly understood, but it is conjectured that the acid salt (used in place of the liquid acid) functions as a diluent making the action of the persulfate more moderate and controlled.

A still further advantage of the pre-admixed powder composition is that it eliminates weighing-out or measuring of more than one ingredient, thus further reducing the possibility of faulty make-up. This is of special importance in the present system where the pH of the treatment solution prepared from the components is critical, as previously mentioned.

In general, the powder compositions of this invention consist in the combination of a persulfate salt that is soluble in water, such salts being selected from the group consisting of ammonium, sodium and potassium persulfates, together with :an acid salt of sulfuric acid likewise soluble in water, such acid salt being selected from the same cation group as in the case of the persulfate, namely ammonium, sodium and potassium. The persulfate and tcid sulfate salts are comminuted if necessary simply to reak up any large agglomerations and are thoroughly admixed to provide a uniform composition. The admixzure is controlled in respect to the amounts of each salt n the final composition, whereby the weight ratio of the acid salt of sulfuric acid to that of the persulfate, computed on the basis of mole equivalents of sodium bisulfate and ammonium persulfate, respectively, ranges from 1:1 to 10:1. The lower ratio just stated, i.e., equal weights of bisulfate and persulfate salt, is a limiting one in order that treatment baths prepared therefrom will be more definitely assured of having a maximum pH value not in excess of 2.0 since this, as explained previously, is critical to successful aluminum desmutting. As will be discussed further, such condition will result inherently when the weight ratios just given are observed upon dissolving the composition in water to make the treatment bath. The higher ratio mentioned above, namely 10:1 is limiting in the sense that the persulfate level here is so low that the speed of reaction of the treatment solution prepared from a mixture of such ratio represents a practical minimum for commercial purposes. Optimum mixture ratios on a gram weight basis are on the order of 5:2 to 5:3, bisulfate to persulfate, with the total weight selected to provide about 70 to 80 grams per liter when put into solution. Such mixtures are completely stable under normal storage, showing no signs of deterioration. Moreover, when these compositions are prepared for use by dissolving them in water, there is no violent exothermic reaction as in the case of adding sulfuric acid to water.

In order that the invention may be better understood, the following examples are given for purposes of illustration.

Example] A dry mixture is prepared containing 50 grams of sodium bisulfate and 30 grams of ammonium persulfate for each liter of deoxidizing solution to be prepared therefrom. The salts when thoroughly mixed can be stored for extended periods of time in appropriate shipping containers without hazard. When it is desired to use the composition for the preparation of a treatment bath in deoxidizing aluminum, the composition is dissolved in water to make the appropriate volume of solution for the weight of material employed. If 80 grams total of the foregoing admixed dry powder composition is used, this should be made up to one liter of solution. This solution will have a pH=l.2 at 25 C.

Aluminum objects having a smutted surface when immersed in the above solution for a period of from seconds to 2 minutes at ambient temperatures of 50 to 90 F., attain a light and smut-free surface which forms an excallent base for further finishing operations including anodizing, painting, bright dipping, dyeing, chromating, etc. It is also found that aluminum objects treated in the above solution show excellent protection against dulling on subsequent-atmospheric exposure.

Example I] In the foregoing Example I, the relative proportion of acid to persulfate salt was selected to give a ratio of 50.grams of the acid salt to 30 grams of persulfate per liter of solution. That ratio may be varied, as indicated previously, to increase the relative amount of persulfate salt. For example, 50 grams of sodium bisulfate and 50 grams of ammonium persulfate, when dissolved in water to make a liter of solution, produces a solution having a pH of 1.10 at 25 C.

Again, aluminum objects having a smutted or highly oxidized surface when immersed in such solution for a period of 25 seconds to 2 minutes at the ambient tem peratures mentioned above, results in complete removal of smut.

4 Example III Aliquot parts of the same powder composition as that prepared in Example I, namely 50 grams of sodium bisulfate and 30 grams of ammonium persulfate, were dissolved in each instance in different volumes of water to show the effect of dilution upon both the solution pH and treatment time. The results are tabulated in Table I.

TABLE I Volume of Solution pH at Time to Remove Solution 25 C. Smut (mm.)

2 lliJGlS l. 35 4 5 liters 1. 65 13% 10 liters 1. 90 25 To show further the effect of different weight ratios of bisulfate and persulfate, a composition was prepared consisting of 0.5 gram of bisulfate and 79.5 grams of persulfate, again making a total dry salt weight of grams. When this is dissolved in water to produce 1 liter of solution, a solution of pH of 2.60 at 25 C. is obtained. Similarly, an admixture of 0.4 gram of bisulfate and 79.6 grams of persulfate were prepared and dissolved as before to produce 1 liter of solution, which had a pH of 3.4 at 25 C.

Aluminum alloy panels treated to produce a smut on the surfaces thereof identical to the samples mentioned above in Example III, were then immersed in these two solutions. The following results were obtained.

Lower proportions of persulfate relative to bisulfate than that given in Example I are useful as for example a ratio of 1 gram of ammonium persulfate to 10 grams of sodium bisulfate. Such a composition (totalling 11 grams) when dissolved in water to produce a liter of solution will produce a solution pH =l.5 at 25 C. Although this is well below the critical pH, because of the low. concentration of the persulfate, the time necessary to effect deoxidizing of aluminum is increased substantially proportionately. Thus a smutted aluminum alloy panel which is completely freed of smut in 30 seconds when immersed in a solution prepared in accordance with Example I, will require soaking in a solution prepared in accordance with this example for a period of from 5 to 10 minutes in order to get comparable results.

The. compositions described above in :Examples I, II, III, and V are suitable for preparation of solutions for desmutting aluminum and aluminum alloys where there is no significant amount of silicon present as an alloying element. By significant amounts, 1% or more is intended. In cases where significant silicon is present in the aluminum alloy, the inclusion of a fluoride compound which is soluble in water is necessary in conjunction with the bisnlfate-persulfate composition. The following is typical of a satisfactory formulation for this purpose.

Example VI A mixture of 50 grams of sodium bisulfate and 30 grams of ammonium persulfate, as in Example I, is prepared. To this is added ammonium bifluoride, which is then thoroughly mixed with the other salts. The amount of the :bifluoride added may range from as little as 0.1 gram to as much as 20 grams for the weights of bisulfate and persulfate given. This represents, on a weight basis from around 0.1% to 20% of the total dry composition. Other soluble inorganic fluorides may be used in place of ammonium. When this dry admixture is dissolved to make one liter of solution, objects of aluminum containing 1% silicon or more, immersed in this solution at room temperature, form highly satisfactory surfaces within a period of 20 seconds to 2 minutes, and are ready, on proper rinsing for further treatment.

Substitutions in the examples above of either sodium or potassium persulfate for the ammonium salt may be made with equally good results. The same applies also to the use of potassium or ammonium bisul-fate in place of the sodium salt. The weights of each should be the gram molecular equivalents of the salts specifically named in the foregoing examples.

While the high solubility of the reaction products of the deoxidizing treatment of aluminum using the foregoing compositions makes rinsing of the aluminum a very simply matter, the compositions may further incorporate a crystalline acid-compatible surfactant, for example, sodium lauryl sulfate, to facilitate run-oif of adherent solution during transfer of the aluminum articles from the deoxidizing bath to a subsequent operation. When using such a cleaning and deoxidizing composition, it is desirable to employ higher solution temperatures than for straight deoxidizing treatment alone. In such cases, operating temperatures of from 7 0 to 160 F. for the solution is recommended.

Processing time for the aluminum articles of course depends on the degree of oxidation of the metal surface and on the concentration of the active agents in the solution employed. In the range of preferred compositions indicated, the solutions produced from them normally provide the desired deoxidizing action after immersion times of 20 seconds to 2 minutes. The upper limit however is not critical as the solutions will not dimensionally affect immersed parts even on prolonged exposure. The lower limit of operability is at least 10 seconds.

What is claimed is:

1. A dry composition for desmutting aluminum and aluminum alloys, which consists essentially of an admixture of dry salts selected from the group consisting of sodium, potassium, and ammonium persulfate, and a soluble acid salt of an acid selected from the group consisting of the sodium, potassium and ammonium acid salts of sulfuric acid, said acid salt of sulfuric acid and said persulfate salt being present, on a mol equivalent basis, in ratios of from 1:1 to 10:1.

2. A dry composition for desmutting aluminum and aluminum alloys as defined in claim 1, which further includes a dry, water soluble salt of a fluoride in amounts of from 0.1% to 20% of the total weight of the dry composition.

3. A dry composition for desmutting aluminum and aluminum alloys which consists essentially, in admixture, of crystalline sodium bisulfate and ammonium persulfate in a weight ratio of about 5:3.

4. A dry composition for desmutting aluminum and aluminum alloys as defined in claim 3, which further includes a dry, water soluble inorganic salt of a fluoride in amount of from 0.1 to 20 grams for each grams of combined sodium bisulfate and ammonium persulfate.

References Cited UNITED STATES PATENTS 3/1958 Newman 25279.3 8/1962 Michaels 156-14 

1. A DRY COMPOSITION FOR DESMUTTING ALUMINIUM AND ALUMINUM ALLOYS, WHICH CONSISTS ESSENTIALLY OF AN ADMIXTURE OF DRY SALTS SELECTED FROM THE GROUP CONSISTING OF SODIUM, POTASSIUM, AND AMMONIUM PERSULFATE, AND A SOLUBLE ACID SALT OF AN ACID SELECTED FROM THE GROUP CONSISTING OF THE SODIUM, POTASSIUM AND AMMONIUM ACID SALTS OF SULFURIC ACID, SAID ACID SALT OF SULFURIC ACID AND SAID PERSULFATE SALT BEING PRESENT, ON A MOL EQUIVALENT BASIS, IN RATIOS OF FROM 1:1 TO 10:1. 